Aqueous dispersions of fluoropolymers are well known in the art as being used for spray application to substrates, followed by drying and baking to form a non-stick coating on the substrate. Such dispersions typically have low viscosities, such as 100 centipoises (cps), to facilitate the spraying operation. Unfortunately, the resultant coating has a thickness limitation, which is that as the dry coating thickness approaches 12 μm, it cracks upon drying and baking, requiring that multiple thinner coats of the dispersion be applied, with intervening drying and baking, in order to build up the crack-free coating thickness desired, e.g., at least 30 μm to provide increased abrasion resistance. Thickeners that decompose and volatilize during baking of the fluoropolymer coating have been added to the aqueous dispersion, to increase the viscosity of the dispersion, with the goal of obtaining thicker one-pass coatings. One disadvantage of thickeners is that they are not approved for food contact in non-stick coating in many countries, notwithstanding the fact that they are designed to volatize during baking. However, as coating thickness increases volatilization is inhibited leaving decomposed thickener in the baked coating. Another disadvantage of thickeners is that they are not present in the baked composition, whereby they do not contribute to the properties of the resultant baked coating. U.S. Pat. No. 6,277,487 B1 discloses the use of such a thickener, which is a polyether, and its blending together with fluoropolymer fine particles, a polyether surfactant, and liquid carrier by stirring to form a uniform dispersion. According to the Examples, the composition has a viscosity of 250 to 280 cps (Brookfield Viscometer, #2 spindle (SP2), operating at 60 rpm) and is applied to a primed substrate by spraying.
There is a need for a thick fluoropolymer coating that can be formed by single-pass coating, without the use of temporary thickener additives, such as the polyether of U.S. Pat. No. 6,277,487, and wherein the coating has widespread governmental approval for food contact.